The quality of an optical signal transmitted via an optical fiber depends upon chromatic dispersion. Accordingly, in long-distance optical fiber transmission, in order to suppress degradation of the waveform of an optical signal due to chromatic dispersion, one or more chromatic dispersion compensators are provided on a transmission path. In this case, each chromatic dispersion compensator is adjusted such that the residual chromatic dispersion is within a dispersion tolerance range at the reception terminal for each path (which will be referred to as “wavelength path” hereafter) via which an optical signal having a corresponding wavelength is to be transmitted. Accordingly, in order to improve the transmission quality of optical signal transmission, it is important to design the dispersion compensation values for the chromatic dispersion compensators.
As a chromatic dispersion compensation design method for an optical network, a method has been proposed in which the compensation values are determined according to a dispersion compensation map. With such a method, in general, the dispersion compensation map is provided for a path between two given points. By setting the dispersion compensation values for the chromatic dispersion compensators in the network according to the dispersion compensation map, such an arrangement is capable of suppressing degradation of the waveform due to chromatic dispersion that occurs in the transmission path via which an optical signal is to be transmitted between the two given points. However, this method has a problem of errors, i.e., the difference between the design value of the chromatic dispersion estimated for an optical fiber used for an optical transmission path and the actual chromatic dispersion, and a problem of errors due to the use of discrete compensation values provided by the chromatic dispersion compensators. The cumulative value of such errors depends upon the wavelength. Furthermore, in recent years, the optical add/drop node (OADM: Optical Add/Drop Multiplexer) and the wavelength cross connect node have been introduced. In such an arrangement, even if the compensation values for the chromatic dispersion compensators are set according to the dispersion compensation map for two given points in the network, in some cases, the optimum chromatic dispersion compensation values are not set for another wavelength path defined between a different pair of given points included entire path between the aforementioned two points or the part of it. This leads to difficulty in providing chromatic dispersion compensation which satisfies a desired dispersion map for all the wavelengths used in the optical network.
FIG. 16 is a diagram for describing a design method for chromatic dispersion compensation according to conventional techniques. An optical network illustrated in FIG. 16 includes optical add/drop nodes A, D, G, and J. Optical repeater nodes B and C are provided between the optical add/drop nodes A and D. Optical repeater nodes E and F are provided between the optical add/drop nodes D and G. Optical repeater nodes H and I are provided between the optical add/drop nodes G and J. Furthermore, each of the nodes A through J includes its own chromatic dispersion compensator.
In the aforementioned optical network, before operation is initiated, a wavelength path 1 is set between the optical add/drop nodes A and J, and a wavelength path 2 is set between the optical add/drop nodes D and G. In this case, the compensation value for the chromatic dispersion compensator provided to each optical node is set to an optimum value for the wavelength paths 1 and 2. Accordingly, the optical signal transmitted via the wavelength path 1 is protected from the effects of chromatic dispersion that occurs in the transmission path from the optical add/drop node A to the optical add/drop node J. The same can be said of the wavelength path 2.
A related technique is described in International Publication Pamphlet No. WO 2005/006604. That is to say, in a chromatic dispersion compensation design method described in International Publication Pamphlet No. WO 2005/006604, the compensation values for the chromatic dispersion compensators provided to each path are determined such that the residual dispersion range at each node due to the paths that arrive at the node is within a permissible residual dispersion range set for each path. Such a method provides a common and formulated optimized chromatic dispersion compensation for a point-to-point transmission system or a ring-structure system.
In the design method illustrated in FIG. 16, the chromatic dispersion compensation design is made only for the wavelength paths 1 and 2. In other words, the design is made without giving consideration to chromatic dispersion in the other paths. Specifically, such an arrangement does not perform chromatic dispersion compensation design for the path between the optical add/drop node A and the optical add/drop node D (which will be referred to as the “path A to D” hereafter), the path A to G, the path D to J, and the path G to J. Accordingly, in a case in which an additional new path is set in such paths, in some cases, the transmission properties of such an additional new wavelength path do not satisfy the permissible level. In the example illustrated in FIG. 16, the transmission properties of the path D to J and the path G to J are lower than the threshold level.
It should be noted that, in order to set such an additional new wavelength path having transmission properties that satisfy the permissible level, there is a need to recalculate the compensation value for the chromatic dispersion compensator included in each optical node. However, such recalculation must be made not only giving consideration to the transmission properties of the additional new wavelength path, but also giving consideration to the transmission properties of the wavelength paths 1 and 2 set previously. This leads to complicated recalculation of the compensation values for the chromatic dispersion compensators.
As one countermeasure against such a problem, a method can be conceived in which the chromatic dispersion compensation is designed beforehand such that the transmission properties satisfy the permissible level for all the paths. However, it is difficult to design an optical network having a complicated topology such that the transmission properties satisfy a uniform permissible level for all the paths. Furthermore, a configuration including optical fibers having high chromatic dispersion properties or chromatic dispersion compensators having high performance leads to an increase in the cost of constructing the network.
It is an object of the present invention to provide a method for designing compensation values for multiple chromatic dispersion compensators provided in an optical network such that the transmission qualities of existing optical paths and additional new optical paths satisfy a predetermined level.